Magnetic-field dependence of photon-echo decays in ruby

Measurements of photon-echo decays in dilute ruby (${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$ concentration of 0.018 at. %) in magnetic fields up to 29.5 kG are reported. Echoes from the ${}_{}{}^4{}_{}{}^{}$ ${\mathit{A}}_2$(-3/2)${\mathrm{\leftrightarrows }}^2$E,E¯(-1/2) and ${}_{}{}^4{}_{}{}^{}$ ${\mathit{A}}_2$(-1/2)${\mathrm{\leftrightarrows }}^2$E,E¯(-1/2) transitions have dephasing times (${\mathit{T}}_2$) that are directly proportional to the ratio of the magnetic field (B) to the temperature (T), for the range 2.5B/TB/T>10 kG/K is observed. Decay times for the ${}_{}{}^4{}_{}{}^{}$ ${\mathit{A}}_2$(-1/2)${\mathrm{\leftrightarrows }}^2$E,E(-1/2) echo are roughly a factor of 3 longer than for the ${}_{}{}^4{}_{}{}^{}$ ${\mathit{A}}_2$(-3/2)${\mathrm{\leftrightarrows }}^2$E,E¯(-1/2) echo. We attribute the lengthening of ${\mathit{T}}_2$ with increasing B/T to the suppression of Cr-Cr spin flip-flops as the population of the upper ${}_{}{}^4{}_{}{}^{}$ ${\mathit{A}}_2$ levels decreases.